This invention relates to a coupling unit of the general type which comprises a hook or a similar member having an at least substantially closed eye, and a substantially U-shaped shackle or U-bolt, which is adapted to be coupled to an end link of a chain and has two opposing shanks or ears which are coupled together by a removable bolt, spindle or the like which extends through registering apertures in the shanks, the mutual distance between the shanks and their shape as well as the cross dimensions of the spindle being so interrelated, that the shackle is connectable only to a link, whose dimensions lie between predetermined, preferably standardized limits.
In the utilization of a chain for lifting and securing purposes as well as in other conjunctions, the chain is as a rule used together with some kind of fitting, such as a hook, an attachment eye member or the like which is connected to the end link of the chain. This coupling together is usually brought about by means of a coupling link, e.g. a link according to the Swedish Pat. No. 211 934, or a shackle, e.g. according to Swedish standard SMS 1577. To give the complete lifting tool, comprising chain, coupling member and fitting (e.g. a hook) a correct maximum working load permitted, the individual maximum working load permitted of each component has to be observed by itself, and the lowest maximum working load permitted will be determinative of the maximum working load permitted of the complete lifting tool. When other persons than experts make such a coupling, it may therefore happen that components having substantially differing maximum permitted working loads become connected together, which entails a great danger of overloading the weakest component and a consequent risk of accidents.